The present invention pertains generally to ophthalmic laser surgery systems and procedures. More particularly, the present invention pertains to a contact lens for use in conjunction with a surgical procedure that allows a surgical laser to be precisely focused at a predetermined location within the cornea of a patient""s eye. The present invention is particularly, but not exclusively, useful for creating a corneal flap that can be subsequently used in a surgical procedure to improve a patient""s vision by altering the shape of the patient""s cornea.
There are many surgical procedures in which it is desirable to be able to focus a laser beam at a predetermined location within a patient""s cornea with precision and accuracy. One such surgical procedure involves the creation of a corneal flap that can be lifted to expose stromal tissue. Once exposed, the stromal tissue can be vaporized using a laser to reshape the cornea. An example of a procedure that uses a laser beam focused at a predetermined location within a patient""s cornea is disclosed in U.S. Pat. No. 4,907,586, which issued to Bille et al. for an invention entitled xe2x80x9cMethod for Reshaping the Eyexe2x80x9d. In greater detail, the above-cited Bille patent discloses the use of a pulsed laser beam for subsurface photoablation of intrastromal tissue. Unlike the excimer laser, the pulsed laser beam, as disclosed by Bille, penetrates corneal tissue and can be focused at a point below the surface of the cornea to photoablate stromal tissue at the focal point. The ability to reach a subsurface location without necessarily providing a physical pathway allows for volumes of stromal tissue having complex shapes to be accurately photoablated, while minimizing the amount of total tissue disrupted.
When considering subsurface photoablation, a general knowledge of the anatomy of the cornea is helpful. In detail, the human cornea comprises various layers of tissue that are structurally distinct. In order, going in a posterior direction from outside the eye toward the inside of the eye, the various layers in a cornea are: an epithelial layer, Bowman""s membrane, the stroma, Decemet""s membrane, and an endothelial layer. Of these various structures, the stroma is the most extensive and is generally around four hundred microns thick. It happens that the healing response of the stromal tissue is generally quicker than the other corneal layers. Because of the relative abundance of stromal tissue and its healing response, stromal tissue is generally selected for removal in refractive correction procedures.
In detail, the stroma of the eye is comprised of around two hundred identifiable and distinguishable layers of lamellae. Each of these layers of lamellae in the stroma is generally dome-shaped, like the cornea itself, and they each extend across a circular area having a diameter of approximately nine millimeters. Unlike the layer that a particular lamella is in, each lamella in the layer extends through a shorter distance of only about one tenth of a millimeter (0.1 mm) to one and one half millimeters (1.5 mm). Thus, each layer includes several lamellae. Importantly, each lamella includes many fibrils which, within the lamella, are substantially parallel to each other. The fibrils in one lamella, however, are not generally parallel to the fibrils in other lamellae. This is so between lamellae in the same layer, as well as between lamellae in different layers. Finally, it is to be noted that, in a direction perpendicular to the layer, each individual lamella is only about two microns thick.
Another important characteristic of the stroma is the strength of the stromal tissue. In greater detail, the strength of the tissue within a lamella is approximately fifty times the strength that is provided by the adhesive tissue that holds the layers of lamellae together. Thus, much less laser energy is required to separate one layer of a lamella from another layer (i.e. peel them apart), than would be required to cut through a lamella. Along these lines, co-pending U.S. patent application Ser. No. 09/783,665, filed on Feb. 14, 2001 by Bille and entitled xe2x80x9cA Method for Separating Lamellaexe2x80x9d discloses a method for finding an interface between layers of lamellae for efficient photoablation. As disclosed in co-pending application Ser. No. 09/783,665 (hereinafter Bille ""665), a wavefront analyzer in conjunction with an ellipsometer can be used to maintain the focal point of a laser beam on an interface between layers of lamellae during creation of a corneal flap for a LASIK type procedure. Use of this technique to photoablate the entire inner surface for a flap has been disclosed in Bille ""665.
A somewhat similar method for creating a LASIK type flap is disclosed in co-pending U.S. patent application Ser. No. 09/997,167, filed on Nov. 28, 2001 by Bille and entitled xe2x80x9cA Method for Creating a Corneal Flapxe2x80x9d. As disclosed in co-pending application Ser. No. 09/997,167, a periphery for a flap can be created using subsurface photoablation along an interface between layers of lamellae. The periphery, in turn, can be used as a starting point to allow layers of lamellae to be mechanically separated from each other along an interface by simply grasping and peeling the flap away from the remainder of the cornea.
In either of these methods wherein photoablation along an interface is desired, the overall movement of the laser focal point is generally along a curved path that is at a substantially constant depth from the anterior surface of the cornea. Thus, it is generally necessary to provide a system to move the laser focal point along this curved path. As the focal point is moving along the generally curved path, a wavefront analyzer and an ellipsometer can be used periodically to verify that photoablation is occurring on an interface between layers of lamellae. When a photoablation response indicates that photoablation is no longer occurring on an interface, a minor adjustment can be made to the depth of the laser focal point to resume photoablation on the interface.
With this in mind, the present invention is focused primarily on providing systems and methods for moving the laser focal point along the curved path (i.e. along paths that are generally parallel to the anterior surface of the cornea). On the other hand, co-pending applications Ser. Nos. 09/783,665 and 09/997,167 provide systems and methods for making minor adjustments to the depth of the laser focal point to maintain the laser focal point on the interface between layers of lamellae. As such, the contents of co-pending application Ser. Nos. 09/783,665 and 09/997,167 are hereby incorporated herein by reference. It follows from the above discussion that the systems and methods for moving the laser focal point along the curved path must be extremely accurate (i.e. accuracy on the order of xc2x12 xcexcm) if these systems are to be used to maintain a laser focal point on an interface between layers of lamellae.
Another factor that must be considered when creating corneal flaps by subsurface stromal photoablation is the elastic compressibility of the lamellae in the cornea. Specifically, it is known that the elastic compressibility of the lamellae varies within the cornea with the elastic compressibility being greatest near the center of the cornea. The consequence of this variation in elastic compressibility becomes significant if the cornea is flattened excessively during subsurface stromal photoablation. During severe flattening of the cornea, the three-dimensional architecture of the lamellae in the cornea becomes distorted. The result of this distortion is that an incision that is made while the cornea is severely flattened changes shape in an unpredictable way when the cornea is relaxed.
Still another factor that must be considered when creating corneal flaps by subsurface stromal photoablation is the beam path of the laser beam. Ideally, all beam paths used to create the flap would be oriented normal to the anterior surface of the cornea to eliminate complications due to refraction of the laser beam at the anterior surface. Unfortunately, typical laser delivery systems are not agile enough to maintain the laser beam on beam paths that are oriented normal to the anterior surface. Thus, for procedures where high precision is required, some compensation must be made for these deviations in beam path due to refraction. Additionally, the optical properties of the cornea, such as corneal density and birefringence, vary from location to location within the cornea. These optical properties can also alter the beam path of a surgical laser beam, and accordingly, it is also desirable to compensate for these deviations in beam path.
In light of the above, it is an object of the present invention to provide systems and methods for creating a corneal flap suitable for use in a corneal reshaping procedure. Another object of the present invention is to provide systems and methods for accurately guiding a laser focal point along a predetermined curved path within the cornea such as an interface between layers of lamellae. It is yet another object of the present invention to provide a contact lens for use in a subsurface stromal photoablation procedure that stabilizes the cornea without upsetting the three-dimensional architecture of the corneal lamellae. It is still another object of the present invention to provide a contact lens for use in a subsurface stromal photoablation procedure that imparts a known radius of curvature to the anterior surface of the cornea to thereby allow a laser focal point to be guided along a path within the cornea relative to the anterior surface of the cornea. Another object of the present invention is to provide systems and methods for accurately guiding a laser focal point along a predetermined path within the cornea that compensate for beam refraction by selectively moving the laser source in a direction parallel to the optical axis of the eye. It is yet another object of the present invention to provide systems and methods for accurately guiding a laser focal point along a predetermined path within the cornea that compensate for variations in the optical properties of the cornea by selectively moving the laser source in a direction parallel to the optical axis of the eye. It is still another object of the present invention to provide a contact lens having a refractive index gradient that compensates for variations in the optical properties of the cornea to thereby allow a laser focal point to be accurately guided along a predetermined path within the cornea. Still another object of the present invention is to provide a contact lens having a refractive index gradient that compensates for beam refraction to thereby allow a laser focal point to be accurately guided along a predetermined path within the cornea. Still another object of the present invention is to provide systems and methods for creating corneal flaps that are easy to use and comparatively cost effective.
The present invention is directed to a system and method for accurately guiding a laser focal point along a predetermined path within the stroma of the cornea. For the present invention, the system includes a contact lens for conforming the anterior surface of a patient""s cornea to a known radius of curvature. In detail, the contact lens has a posterior surface and an anterior surface. Preferably, the contact lens has a substantially constant thickness with the anterior surface being spaced from the posterior surface by a distance of approximately 0.2 mm. Importantly, the posterior surface of the contact lens has a substantially uniform radius of curvature, Rlens, that is approximately 8.3 mm.
For the present invention, the contact lens is preferably made of a clear material, such as plastic to thereby allow a surgical laser beam to be passed through the contact lens. In a first embodiment of the present invention, the contact lens has a substantially uniform index of refraction that closely matches the index of refraction of a typical cornea to minimize refraction at the interface between the contact lens and the cornea. An exemplary contact lens for this first embodiment is prepared having a uniform index of refraction of approximately 1.4 (as compared to a typical index of refraction for the human cornea which is approximately 1.37).
In accordance with the present invention, the contact lens in mounted in a suction ring. In use, the posterior surface of the contact lens is gently pressed again the anterior surface of the cornea until the anterior surface of the cornea conforms to the posterior surface of the contact lens. Next, scleral suction is applied via the suction ring to hold the contact lens against the cornea. Because the anterior surface of a typical cornea has a radius of curvature that is approximately 7.8 mm, the anterior surface of the cornea will conform to the posterior surface of the contact lens (Rlens=8.3 mm) when the posterior surface of the contact lens is pressed against the cornea. If desired, the suction ring can be attached to a fixed structure, such as the laser source base, to stabilize the eye during the laser procedure. Importantly, this slight flattening of the cornea causes minimal discomfort to the patient and does not upset the three-dimensional architecture of the corneal lamellae.
In accordance with the present invention, a laser source is provided to generate a surgical laser beam. Included in the laser source is a cutting lens to focus the laser beam to a subsurface focal point within the cornea for the purpose of photoablating stromal tissue. The laser source is positioned relative to the patient""s eye to allow a laser beam to be generated and directed along a first beam path that is collinear with the optical axis of the eye (hereinafter referred to as the z-axis). It is to be appreciated that this first beam path is substantially normal to the anterior surface of the contact lens at the incident point where the first beam path passes through the anterior surface of the contact lens.
For the present invention, the laser source is mounted on a scanning mechanism to allow the focal point of the laser beam to be scanned along a predetermined path within the cornea. In greater detail, the scanning mechanism is capable of moving the laser source within a plane that is normal to the optical axis. As the laser source moves within the plane, the laser beam is placed on successive beam paths, with each beam path passing through a different incident point on the anterior surface of the contact lens.
Because the anterior surface of the contact lens is curved, each point on the surface defines a unique surface normal. With the cooperation of structure described above, each off-axis beam path passes through the anterior surface of the contact lens at an angle to the surface normal that is defined at the point of incidence. Because of this angle, a laser beam traveling on an off-axis beam path will be refracted at the anterior surface of the contact lens. However, the scanning mechanism does provide some additional tilting of the laser beam when the laser source is positioned at a distance from the z-axis. More specifically, as the laser source is moved radially away from the z-axis, the tilt of the laser beam relative to the z-axis increases. Typically, this tilting occurs at a rate of approximately 1/mm of radial distance that the laser source is moved from the optical axis. More specifically, at the outer periphery of the cornea, the laser beam has moved radially about 4 mm from the z-axis and has tilted through an angle of approximately 3xc2x0 from the z-axis.
In accordance with the present invention, the scanning mechanism can also selectively move the laser source in a direction parallel to the z-axis. It is to be appreciated that movements of the laser source in a direction parallel to the z-axis will result in corresponding movements of the focal point of the laser beam in a direction parallel to the z-axis. As the focal point moves along a curved path within the cornea, the z-axis movement of the focal point allows the system to control the depth of the focal point (measured from the anterior surface of the cornea). The magnitude, z, of the z-axis movement required to control the depth of the focal point as the focal point moves along a curved path includes three components; z1, z2 and z3. The first component, z1, is geometrical and does not include the effects of refraction. This first component, z1 is dependent upon the shape of the contact lens and any contribution due to the tilt of the laser beam relative to the optical axis. The second component, z2, compensates for refraction that occurs at the surfaces of the contact lens. The third component, z3, compensates for refraction caused by the anatomical configuration of the cornea.
For example, consider the case where photoablation along an interface between two lamellae is desired. With the anterior surface of the cornea conforming to the contact lens (Rlens=8.3 mm), it is to be expected that an interface between lamellae will also lie along a curved path having a radius of curvature of about 8.3 mm. Thus, a z1 movement of the laser focal point is required to maintain the focal point on the interface between lamellae layers during movement of the focal point along the interface. Specifically, to maintain the focal point at a constant depth from the anterior surface of the cornea (i.e. depth into the cornea), a z1 movement of approximately 1.5 mm must be made as the focal point moves from a point on the z-axis to a point approximately 4 mm from the z-axis near the periphery of the cornea.
In addition to the z, movements required to follow the radius of curvature of the lens with the focal point, z2 movements can be used to compensate for the effects on focal point depth from refraction that occurs at the surfaces of the contact lens. The magnitude of the z2 correction varies in magnitude from zero on the optical axis to about 7 xcexcm at a point approximately 4 mm from the z-axis near the periphery of the cornea.
As indicated above, the third component, z3, compensates for refraction caused by the anatomical configuration of the cornea. Specifically, it is known that the density and birefringent properties of the cornea vary from location to location within the cornea. As the focal point moves along a path within the cornea, variations in the density and birefringent properties of the cornea will effect the depth of the focal point. These variations in the density and birefringent properties of the cornea, however, can be compensated by z3, movements. More specifically, the density of the cornea can be measured and mapped using wavefront analysis and the birefringent properties of the cornea can be measured and mapped using an ellipsometer. The maps can then be used to calculate z3 movements that will compensate for these variations in corneal properties. Typical values for a z3 correction will be in the range of 5-8 xcexcm.
In another embodiment of the present invention, a contact lens having a non-uniform index of refraction is used to compensate for effects on focal point depth from refraction that occurs at the surfaces of the contact lens and variations in corneal properties. Thus, for this embodiment, the z2 and z3 movements of the laser source can be reduced or eliminated. For the present invention, the non-uniform index of refraction can be accomplished by ion implantation of the plastic lens using masking techniques. To compensate for effects on focal point depth from refraction that occurs at the surfaces of the contact lens, a contact lens having an index of refraction profile is used. Since this refraction is characteristic of the lens shape, the index of refraction profile will be the same for all lenses having the same shape. Specifically, for this embodiment, the portion of the contact lens that is on the z-axis will have the highest index of refraction while the periphery of the contact lens will have an index of refraction that is reduced by about 3 percent.
To compensate for effects on focal point depth due to the anatomical configuration of the cornea, a corneal mapping of the density and birefringent properties of the cornea is first prepared as described above. With the mapping, a contact lens can be selectively altered via ion implantation to compensate for the variations in corneal properties. Thus, the required contact lens will differ from patient to patient. However, it is contemplated that all corneas can be classified into about twenty anatomically similar groups. Thus only about twenty different contact lenses are required to compensate for the anatomical configuration of the cornea with reasonable accuracy.